The problem of determining the pose of a sensor with respect to the end-point (or tool flange) frame of a robot based on experimental measurements using the sensor, is generally referred to as the "sensor mount registration problem." This problem was first addressed in Y. C. Shiu & S. Ahmad, "Finding the Mounting Position Of A Sensor By Solving A Homogeneous Transform Equation of the Form AX=XB", Proc. 1987 IEEE Robotics & Automation Conf., pp. 1666-1671. A number of other publications have since been published on the topic which deal primarily with alternative mathematical approaches to the solution of the problem, see J. C. K. Chou & M. Kamel, "Quaternions Approach to Solve the Kinematic Equation of Rotation A.sub.a A.sub.x =A.sub.x A.sub.b of a Sensor Mounted Robotic Manipulator", and F. C. Park & B. J. Martin, "Robot Sensor Calibration: Solving AX Equals XB On The Euclidean Group", IEEE Trans Robotics & Automation, Vol. 10 No. 5, 1994, pp717-721. However, the techniques disclosed by these references assume that the sensor is capable of measuring pose completely.
Everett et al. have proposed a more generalised technique, capable of solving the generalised sensor registration problem, as long as the sensor was capable of measuring at least one position variable and two independent components of the orientation variables, see L. J. Everett & L. E. Ong, "Solving The Generalised Sensor-Mount Registration Problem", Proc 1991 ASME Winter Annual Mtg., DSC V. 29. (ASME) pp7-14. Using a specialised calibration fixture attached to the tool flange, they showed that the tool transformation could be obtained using a rudimentary sensing procedure.
U.S. Pat. No. 5,457,367, entitled "Tool Center Point Calibration Apparatus And Method", discloses a method for calibrating the position of the tool center point (TCP) of a spot welding gun. The profile of each gun tip is measured by moving the tip back and forth across a light beam, and detecting when the light beam is broken. Once the center of the gun tip has been found, its position with respect to the robot end-of-arm face plate (or tool flange) is estimated, based on the specified and measured position of the light beam in the robot workspace. Two rotations with respect to this found position then enable the position of the gun tip with respect to the tool flange to be computed. This procedure is repeated for the second gun tip. This application is substantially different from the proposed method in that no end-of-arm sensor with which to correct the tool center point placement on-line is considered, and the technique itself requires a search procedure.
U.S. Pat. No. 5,297,238, entitled "Robot End-Effector Terminal Control Frame (TCF) Calibration Method And Device", discloses a method whereby a two dimensional calibration template is viewed by a camera mounted on the end-effector, from a near-normal orientation with respect to the calibration surface. The robot is moved parallel to the calibration surface to three different positions, and an image of the template is taken from each position. By analysing these three images, the X and Y translation components of the TCF, as well as the rotation about the TCF Z axis can be calculated. The TCF pose thus measured is incomplete, with the Z translation component unknown, and the prerequisite near-normal orientation with respect to the calibration surface effectively setting the pitch and yaw angles to zero (ie. not measured). This method can therefore not be used to measure the complete pose of the sensor with respect to the reference feature.
U.S. Pat. No. 5,329,469, entitled "Calibration Method For A Visual Sensor", discloses a method whereby a "dot pattern" on a fixture attached to the robot is viewed by one or more stationary cameras. Based on image analysis of the dot patterns for two robot positions, the position of the fixture with respect to the fixed camera reference frame is established. The objective is to calibrate the fixed camera with respect to the workspace of the robot, so that the camera may subsequently be used to determine the position of a workpiece that is to manipulated by the robot. This a very different application in that calibration of the robot itself is not considered. Indeed the technique is based on the presumption that the position (in robot workspace coordinates) of the fixture that the robot manipulates is exactly known.
U.S. Pat. No. 5,471,312, entitled "Automatic Calibration Method", discloses a method whereby a dot pattern (which may be three dimensional) is attached to the robot and viewed by up to eight stationary cameras, to obtain the spatial position of the dot pattern with respect to a fixed reference frame. The objective is to determine the relative transformation between the camera reference frame(s) and the robot work frame, so that subsequently, a workpiece position inaccuracy detected by the camera(s) may be converted into a robot correction. This technique also enables multiple robots to be calibrated with respect to the same reference frame, and is a logical extension of U.S. Pat. No. 5,329,469. However, this technique is not concerned with end-of-arm sensing or tool frame calibration, and requires a jig to be fitted to the robot.